Most of piezoelectric ceramic materials currently in practical use contain lead as typified by lead titanate (PT) and lead zirconate titanate (PZT). The lead-containing piezoelectric ceramic materials however raise a concern about the environmental effects of their lead components. Further, the lead-containing piezoelectric ceramic materials have a Curie point (Curie temperature) of about 200 to 500° C. and lose their piezoelectric properties at temperatures higher than or equal to the Curie point so that it is difficult to use the lead-containing piezoelectric ceramic materials for piezoelectric ceramic sensors whose operating temperatures are generally 500° C. or higher. There is a need for lead-free piezoelectric ceramic usable at 500° C. or higher with less environmental effects and no loss of piezoelectric properties.
As such lead-free piezoelectric ceramic, bismuth layer-structured ferroelectric materials Na0.5Bi4.5Ti4O15 (NBT) are known. (See Patent Publications 1 and 2 and Non-Patent Publications 1 and 2.) The bismuth layer-structured ferroelectric materials NBT are expected as lead-free piezoelectric ceramic usable under high-temperature conditions due to the fact that the Curie point of the bismuth layer-structured ferroelectric materials NBT is about 670° C. and is higher than the Curie point of the PT and PZT materials.
Patent Publication 1: Japanese Laid-Open Patent Publication No. S50-67492
Patent Publication 2: Japanese Laid-Open Patent Publication No. H11-29356
Non-Patent Publication 1: “Piezoelectricity in Ceramics of Ferroelectric Bismuth Compound with Layer Structure”, S. Ikegami and I. Ueda, Japanese Journal of Applied Physics, 13 (1974), p. 1572-1577
Non-Patent Publication 2: “Grain-Oriented and Mn-Doped (NaBi)(1-x)/2CaxBi4Ti4O15 Ceramics for Piezo- and Pyrosensor Materials”, T. Takenaka and K. Sakata, Sensor and Materials, 1 (1988), p. 35-46
The bismuth layer-structured ferroelectric material Na0.5Bi4.5Ti4O15 (NBT) has a high Curie point and high heat resistance but a low piezoelectric distortion constant (d33) and low mechanical quality factor (Qm). It is difficult to use the bismuth layer-structured ferroelectric materials Na0.5Bi4.5Ti4O15 (NBT) for resonators and sensors as the resonators and sensors require a high piezoelectric distortion constant and high mechanical quality factor. Although it is known that the piezoelectric distortion constant of anisotropic crystal material like the bismuth layer-structured ferroelectric material NBT can be improved by orienting the material into a specific crystal direction, such a crystal orientation technique requires hot press treatment etc. so that the production process of the material becomes complicated to cause an increase in production cost.